Adaptation of plants to the changing environment requires that sensing of external stimuli be linked to the mechanisms of morphogenesis. The Arabidopsis TCH genes are rapidly and strongly regulated in expression by a variety of environmental stimuli. The physiological relevance of TCH gene induction is not clear. We identified TCH4 as a xyloglucan endotransglycosylase (XET) by sequence similarity and enzyme activity. XET modifies xyloglucan, an important component of dicots cell wall, which is a fundamental determinant of cell shape and plant form. Therefore TCH4 product is very likely to affect the cell wall mechanical properties, and the magnitude and direction of cell expansion. The expression pattern of TCH4-GUS fusion gene and the pattern of endogenous TCH4 protein localization revealed that TCH4 is likely expressed in cells that are undergoing cell extension and cells undergoing cell wall modification. In addition to the developmental regulation, the expression of TCH4 is upregulated in plants subjected to environmental stimuli such as touch, darkness, heat shock, cold shock, and plant growth regulators, auxin and brassinosteroid. The function of TCH4 as an XET and its unique regulation property indicate that TCH4 may underlie plant morphogenetic responses to the environmental cues and to the endogenous hormones. A 1 kb TCH4 region was determined to be able to confer the inducibility of expression to the GUS reporter gene for all stimuli tested. Further dissection of the 1 kb TCH4 sequences revealed that the cis-acting elements conferring the inducibility in response to different stimuli are separable. The TCH4 regions conferring TCH4 tissue-specific expression was also determined. The TCH4 expression regulated by environmental, hormonal and developmental factors is regulated through a complex collection of cis-regulatory elements and may occurs through both transcriptional and post-transcriptional mechanisms. Genomic Southern analysis indicates that there is a TCH4-related gene family in Arabidopsis. These genes encode unique XET-related proteins and show differential regulation by environmental and hormonal stimuli. Elucidation of the functions and regulation of this gene family will most likely lead to an insight into the role of the plant cell wall-modifying enzymes in the morphogenesis of plant growth and development responding to the environment
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